The infrared spectrum covers a range of wavelengths longer than the visible wavelengths, but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to the energy of a wavelength within some particular portion of the infrared region.
Heated objects generate radiant energy having characteristic wavelengths within the infrared spectrum. Many current infrared image detection systems incorporate arrays with large numbers of discrete, highly sensitive detector elements, the electrical outputs of which are connected to processing circuitry. By analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and track sources of infrared radiation. Though the theoretical performance of such contemporary systems is satisfactory for many applications, it is difficult to construct structures that adequately interface large numbers of detector elements with associated signal processing circuitry in a practical and reliable manner. Consequently, practical applications for contemporary infrared image detector systems have necessitated further advances in the areas of miniaturization of the detector array and accompanying circuitry, of minimization of circuit generated noise that results in lower sensitivity of the detected signal, and particularly of improvements in the reliability and economical production of detector arrays.
Contemporary arrays of detectors, useful for some applications, may be sized to include 256 detector elements on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.009 centimeters on a side, with 0.00127 centimeters spacing between detectors.
Infrared detector arrays are commonly fabricated from single crystalline layers of an infrared radiation-absorbing material. Layer thicknesses of approximately ten microns are typical. These single crystalline layers are formed by epitaxial crystal growth upon a single crystalline wafer substrate. The substrate must have a closely matching crystal lattice structure to provide epitaxial layer growth. The substrate, which conventionally faces the infrared radiation, must then be transparent to those wavelengths of infrared radiation which are to be detected, i.e., from the surface of the detector in contact with the substrate. Such substrate-side irradiation is used because detector element electrodes and a conductive grid are fabricated upon the surface of the detector layer. The electrodes and conductive grid cannot be formed beneath the detector because a continuous single crystalline substrate surface is required for epitaxial growth.
Mercury cadmium telluride (HgCdTe) can be used to detect short to long wavelength infrared radiation. An epitaxial layer of HgCdTe can be grown upon cadmium telluride (CdTe) or upon CdTe on sapphire or on gallium arsenide (GaAs). It is difficult to obtain a CdTe wafer or layer substrate that results in a high performance HgCdTe detector array.
Difficulty is encountered in attempting to obtain large area wafers of CdTe because it is relatively soft and fragile. It is difficult for such wafers to withstand the many steps encountered in the fabrication process. These processes include expitaxial growth of the HgCdTe layer and the formation of p-n junctions, a conductive grid and insulator isolated electrodes on the HgCdTe layer.
Whether a wafer or an epitaxially grown layer of CdTe is used as the substrate for the epitaxial growth of HgCdTe, there are problems of chemical purity and of crystallographic uniformity that can adversely affect the quality of the HgCdTe layer for fabricating the infrared detector elements. These difficulties and problems in the prior art have to date resulted in extremely low detector array fabrication yields and limited the arrays to small areas.